Clothier in Rialto ruined (6)
I believe the answer is:
tailor
'clothier' is the definition.
(thesaurus)
'rialto ruined' is the wordplay.
'ruined' indicates an anagram.
'rialto' anagrammed gives 'TAILOR'.
'in' is the link.
(Other definitions for tailor that I've seen before include "Fit" , "Maker of men's suits" , "One makes fitted clothes" , "Maker of garments" , "One who makes garments" .)